As I already posted in the other thread, I released a tiny tool for Windows to access the QuickTime AAC encoder from the command line. Now some important features, gapless support and pipe encoding, have been added, so I've decided to create a dedicated thread for this software.

There is no technical reason to expect CVBR to be better than TVBR (especially not at high bitrates*). Conceptually TVBR is superior and CVBR is just TVBR with braces. If the TVBR implementation was somehow flawed, it would rather show at lower bitrates (as in the 2010 listening test). The constraint could limit the encoder from dropping too low (but be limited at the same time to scale up where needed).

You'll find explanations of CVBR from Apple's developer documentation, if you use the forum's search function.

In my opinion the sole purpose of the CVBR mode in iTunes is prevention of consumer confusion. They don't want people calling in why their remastered 1940 album comes out at ~90kbit/s when they had chosen 256 kbit/s average. When there's just not that much information to preserve, the TVBR encoder won't hesitate to do it. The CVBR encoder would instead output a higher bitrate and please consumer prejudice (without audible benefit). Consumers are happy, Apple is happy (less support calls): win/win.